As colour television was introduced in order to maintain compatibility with black-and-white requirements, chrominance information was superimposed on the black-and-white intensity signal in the form of a phase and amplitude modulated sub-carrier signal. The intensity (luminance) signal, having a bandwidth of 4 Megahertz but having most of its power in the lower portion of the spectrum, did not suffer appreciably from the 1.5 Mhz wide chrominance signal centred around a frequency of 3.579545 Mhz, the so-called colour sub-carrier. However, to minimize the interference caused by the colour sub-carrier with the basic luminance information signal, it was necessary for the phase of the colour sub-carrier to appear inverted with respect to the luminance signal from frame to frame. Thus the interference, in the form of a ripple superimposed on the intensity pattern of picture lines, is averaged out due to the persistance of human vision.
Such an apparent phase inversion was simply achieved by choosing a colour sub-carrier frequency which is a half odd multiple of the frame frequency. Given a frame frequency of 29.9770026 per second (close enough to the original monochrome frame rate of 30 frames/second) the colour sub-carrier was chosen to be exactly 119437.5 times that, which yields the sub-carrier frequency of 3.579545 Mhz. A more rigorous description of what actually happens is that due to this half odd-multiple relationship the peaks of energy in the colour sub-carrier fall between the peaks in the luminance signal spectrum, and so are of minimum visibility when viewed on a monochrome receiver. This and a related succinct discussion of the NTSC colour format is given in part of a paper by John O. Limb et al titled Digital Coding of Colour Video Signals--A Review and published November 1977 in IEE (Institute of Electrical and Electronics Engineers) Transactions on Communications, Vol. Com-25, No. 11, at pages 1353 to 1355.
To summarize, this apparent phase shift of the colour sub-carrier between successive frames is a result of the half odd-multiple relationship between sub-carrier and frame frequencies.
A problem, however, arises when a single video frame is stored for viewing as a still picture. Whether a frame is stored in a VTR (Video Tape Recorder) with "Stopped frame" capability or in a single frame digital image store, in repetitive retrieval of the stored frame the same information is repeated over and over. Consequently, the reference colour sub-carrier burst always appears in the same fixed phase relationship with respect to all other elements of the composite signal, horizontal synchronization pulse and all. What this actually means is that an abrupt phase change of 180.degree. occurs in the chrominance signal, including the reference sub-carrier burst, at the beginning of each frame. Since this burst is used as reference for locking the TV monitor's internal sub-carrier oscillator, the oscillator must adjust its output phase by 180.degree. to maintain phase synchronization with the incoming signal in order to decode the chrominance signal correctly. This however typically takes a few milliseconds because such oscillators are designed to have a very tight phase lock (high inertia) since they must maintain their phase without correction for the duration of a full line after the short period of the reference burst at the beginning of every line. The long period required for the reference oscillator to lock in on the new phase would cause a good portion of the picture (frame) to be unacceptably distorted in colour.